During the last few decades, the microelectronics industry has succeeded in building increasing amounts of computing power into smaller and denser device geometries on integrated circuit (IC) chips. The advent of dense, large-scale integration has favored the metal-oxide-field-effect transistor (MOSFET) over the bipolar transistor among the devices used in ICs. In many cases there are practical advantages to making the "metal" electrode in the MOS device of poly-silicon. Critical to the development, production and final performance of advanced IC's is the precise control of the poly-silicon layer thickness and the doping density. The poly-silicon layer is typically sandwiched between two SiO.sub.2 layers, i.e., a thermal SiO.sub.2 layer and a cap SiO.sub.2 layer.
Recent developments in microelectronics manufacturing have emphasized highly flexible single-wafer poly-silicon processes, effected in cluster tools. In current practice, process monitoring and control is provided by post-fabrication metrology, performed outside the cluster tool, for the evaluation of film thickness, doping density, uniformity, and defects. The lack of in-line diagnostics, i.e., of data obtained in real-time from within the cluster tool, results in higher manufacturing costs due primarily to the high proportion of off-spec wafers that are produced, excessive material consumption, and undue personnel requirements. The long turnaround times imposed by off-line metrology also result in lower yields and slower learning curves for new processes and products. In order to fully benefit from the implementation of the cluster tool concept, more and better in-line and in situ process monitoring and control instrumentation is needed.
Due to the complexity of the thin film materials utilized in modern electronic, magnetic, and optical devices, in-line process monitoring is difficult to achieve. Among the many problems that are associated with present techniques are the following: 1) Commercial processes currently rely upon reflectance measurements using visible or near infrared light, making in-line analysis of, in particular, polysilicon problematic. This is so because the near-IR and visible optical properties (i.e., n and k) of silicon are highly dependent upon crystallinity, meaning that short wavelength measurements cannot reliably determine accurate thicknesses for as-deposited poly-silicon films, which often contain sample-dependent, spatially varying mixtures of amorphous and poly-crystalline phases. 2) Visible wavelength techniques are insensitive to doping density, and other standard techniques for doping measurements, such as secondary ion mass spectroscopy (SIMS), are destructive. 3) The current techniques are incompatible with wafer-by-wafer process monitoring and control, since they cannot directly determine the poly-silicon layer thickness on a product wafer which has only thin (.about.100 .ANG.) thermal SiO.sub.2 or Si.sub.3 N.sub.4 layers. 4) Current techniques cannot determine the thickness of a thermal SiO.sub.2 layer, which is covered by layers that are opaque in the visible wavelength range; furthermore, the presence of a cap SiO.sub.2 layer can interfere with measurement of the thermal SiO.sub.2 layer underlying a poly-silicon film. 5) Film properties on patterned wafers can presently be determined by using a microscope objective to measure small, uniformly coated regions on the pattern, but this is expensive and time-consuming, and is subject to fundamental limitations imposed by diffraction and optical throughput. 6) Current large spot measurement techniques are not suitable for patterned wafers, because they require uniform coverage of the film stack across the entire measurement spot (i.e., blankets). 7) And finally, current techniques require high-speed, precise stage motion, pattern recognition, and auto-focusing.
U.S. Pat. No. 5,392,118, to Wickramasinghe, discloses a method for measuring a trench depth parameter of a material, which method comprises propagating source radiation around a trench and through the material, and analyzing a characteristic variation of an interference signal as a determinant of the trench depth parameter. The interference signal developing as radiation from a base of the trench interferes with radiation propagated from a top surface of the material.
U.S. Pat. No. 5,403,433, to Morrison et al., provides a method and apparatus which permit in situ determinations to be made of the temperature and optical constants of a substrate surface that is being treated, by measurements of radiance, reflectance, and transmittance. These determinations in turn provide, at any given instant during processing, compositional and other information, thereby affording highly effective feedback control of the processing conditions. The apparatus comprises an integrated, small, and relatively inexpensive instrument for process monitoring.
In U.S. Pat. No. 5,469,361, Moyne describes a generic cell controlling method and apparatus for a computer-integrated manufacturing system, which accepts manufacturing operation commands to perform a selected manufacturing operation on a selected manufacturing tool. The sequence of generic steps to be performed by the manufacturing tool is determined, in order to implement the selected manufacturing operation, and each step is used to generate operational instructions for the tool controller.
In accordance with Nishizawa et al. U.S. Pat. No. 5,587,792, the thicknesses of respective layers of a thin multi-layer film of submicron thicknesses can be nondestructively measured exactly and stably without direct contact. An interference waveform dispersion spectrum of light reflected from the multi-layer film is compared to a waveform obtained by numerical calculation using an optical characteristic matrix. Respective layer thickness values obtained from the calculated analysis of the spatial interference waveform are subjected to waveform fitting with actually measured values. The theoretical interference spectrum is recalculated while changing approximate values of the layer thicknesses until a match is obtained to obtain precise respective layer thicknesses.
U.S. Pat. No. 5,604,581, to Liu et al., provides a method by which the thickness and the free carrier concentration of at least one layer of a structure are determined. An exposed surface of the structure is irradiated using spectral radiation, and the measured reflectance spectrum is compared to a calculated spectrum. Using algorithms that include terms representative of complex refractive indices, layer thickness, dielectric constants, and free carrier concentrations, values are iteratively assigned to the thickness and free carrier concentration parameters so as to produce a best fit relationship between the compared spectra, and to thereby determine those parameters.
In an article entitled "Real-Time Measurement of Film Thickness, Composition, and Temperature by FT-IR Emission and Reflection Spectroscopy," Solomon, P. R., Liu, S., Rosenthal, P. A., and Farquharson, S. (Semiconductor Characterization--Present Status and Future Needs, Ed. Bullis, Seiter and Diebold, AIP Press, pp 545-548, 1996) discuss the methodology, hardware, and software used to perform on-line or at-line monitoring of thin film parameters, i.e., thickness, temperature, and composition (wavelength dependent dielectric function, doping density, impurities, etc). Measurements of combined thickness and composition are made using Fourier transformed infrared reflection spectroscopy.